There is great interest in engineering surfaces to reduce the flow resistance of a liquid on the surface. Some structured surfaces having nanometer- or micron-sized raised features have promise in applications ranging from the transport of a liquid through a microfluidic channel, to reducing the drag of a vessel traveling through a liquid. However, a number of problems must be overcome before the full benefit of these surfaces can realized.
One problem is that the flow resistance of a liquid on a structured surface can vary dramatically with the pressure of the liquid. If the pressure of the liquid increases, then the liquid will penetrate to a greater extent into the structured surface, thereby increasing the flow resistance of the liquid on that surface. Alternatively, if the pressure of the liquid decreases, then the liquid will penetrate to a lesser extent into the structured surface, thereby decreasing the flow resistance. Flow resistance can also increase when the diffusion of air out of the liquid is sufficient to form air bubbles on the structured surface. For instance, the formation of air bubbles on the interior walls of a microfluidic channel can significantly impede the flow of liquid through the channel.
Embodiments of the present invention overcome these deficiencies by providing an apparatus having a structured surface with improved pressure stability, which thereby provides better control over flow resistance, as well as methods of using and manufacturing such an apparatus.